<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    body>div{
      width: 200px;
      /*div不给高度时高度默认为内容的高度*/
      border: 1px solid red;
      overflow: hidden;
    }
    div>div{
      width: 50px;
      height: 50px;
    }
    #d1{
      background-color: red;
      float: left;

    }
    #d2{
      background-color: green;
      float: left;
    }
    #d3{
      background-color: blue;
      float: left;
    }
    p{
      width: 100px;
      height: 100px;
      background-color: pink;
      margin: 0;
    }
    ul{
      list-style-type: none;/*去掉列表项*/
      margin: 0;
      padding: 0;
      overflow: hidden;/*解决高度为0的显示异常*/
    }
    li{
      float: left;
      margin-right: 20px;
    }
  </style>
</head>
<body>
<ul>
  <li><a href="">首页</a></li>
  <li><a href="">免费课</a></li>
  <li><a href="">直播课</a></li>
  <li><a href="">线上班</a></li>
  <li><a href="">线下班</a></li>
</ul>
<div>
  <div id="d1"></div>
  <div id="d2"></div>
  <div id="d3"></div>
</div>
<p></p>
</body>
</html>